Доказать равенство.
cos20-sin50=sin10

cos 20 - sin50 = sin 70-sin50 = 2.sin(70-50)/2 . cos (70+50)/2 = =2.sin10 . cos60= 2. sin10 . 1/2 = 2.(1/2).sin10 = sin10(sin x - sin y =2. sin(x-y)/2.cos (x+y)/2 )